They have to care in some circumstances unfortunately, if they are aware of an IP infringement and don't action it, it can be used as evidence further down the line for bigger infringement cases.
Trademark is the only form of intellectual property (IP) for which selective enforcement is a problem. Presumably the name of the game is trademarked but the images & audio are covered by copyright law, not trademark.
IANAL but if Blizzard is aware of this copyright infringement and do not act on it, the developer can go for an acquiescence defence. If successful, Blizzard surrender their rights in that particular project.
This could potentially have an effect on the value of their IP which can give leverage to potential acquirers/investors.
No. There is no such defense in U.S. copyright cases. Again, only trademark requires them to pursue infringement to maintain their protection, and by extension, value.
To be honest, StarCraft is probably protected in most countries and don't believe that it is hard for Blizzard to find a local lawyer willing to take the case if the want to.
Please never take "not from the US so what" as a credible form of protection against any US actor, be it corporate or govermental.
There's many ways someone can still screw you, starting from making your travel to the US tricky to much worse, depending on the circumstances and what international agreements allow.
Blizzard can send a DMCA takedown to github and github must ASAP take down the repo regardless of if there was actual copyright infringement. I've seen github developers send DMCA requests to take down other developers repos before...